Diabetes is a major public health concern because of its increasing prevalence and associated health risks. The disease is characterized by metabolic defects in the production and utilization of carbohydrates which result in the failure to maintain appropriate blood glucose levels. Two major forms of diabetes are recognized. Type I diabetes, or insulin-dependent diabetes mellitus (IDDMT1 DM), is the result of an absolute deficiency of insulin. Type II diabetes, or non-insulin dependent diabetes mellitus (NIDDMT2DM), often occurs with normal, or even elevated levels of insulin and appears to be the result of the inability of tissues and cells to respond appropriately to insulin. Aggressive control of NIDDM T2DM with medication is essential; otherwise it can progress into β-cell failure and insulin dependence.
Glucagon is a twenty nine amino acid peptide which is secreted from the α cells of the pancreas into the hepatic portal vein thereby exposing the liver to higher levels of this hormone than non-hepatic tissues. Plasma glucagon levels decrease in response to hyperglycemia, hyperinsulinemia, elevated plasma non-esterified fatty acid levels and somatostatin whereas glucagon secretion is increased in response to hypoglycemia and elevated plasma amino acid levels. Glucagon, through activation of its receptor, is a potent activator of hepatic glucose production by activating glycogenolysis and gluconeogenesis.
The glucagon receptor is a 62 kDa protein that is activated by glucagon and is a member of the class B G-protein coupled family of receptors. Other closely related G-protein coupled receptors include glucagon-like peptide-1 receptor (GLP-1), glucagon-like peptide-2 receptor (GLP-2) and gastric inhibitory polypeptide receptor. The glucagon receptor is encoded by the GCGR gene in humans and these receptors are mainly expressed in the liver with lesser amounts found in the kidney, heart, adipose tissue, spleen, thymus, adrenal glands, pancreas, cerebral cortex and gastrointestinal tract. Stimulation of the glucagon receptor results in activation of adenylate cyclase and increased levels of intracellular cAMP.
Reports have indicated that an uncommon missense mutation in the GCGR gene is correlated with diabetes mellitus type 2 and one reported inactivating mutation of the glucagon receptor in humans causes resistance to glucagon and is associated with pancreatic α-cell hyperplasia, nesidioblastosis, hyperglucagonemia and pancreatic neuroendocrine tumors. In rodent studies with GCGR knockout mice and mice treated with GCGR antisense oligonucleotides the mice exhibited improved fasting glucose, glucose tolerance and pancreatic β-cell function. In both healthy control animals and animal models of type 1 and type 2 diabetes, removal of circulating glucagon with selective and specific antibodies has resulted in a reduction of the glycemic level. More specifically, treatment of both mice and cynomolgus monkeys with GCGR-antagonizing antibodies (mAb B and mAb Ac) has been shown to improve glycemic control without causing hypoglycemia. Recent mice studies have further shown that antagonism of the glucagon receptor results in improved glucose homeostasis through a mechanism which requires a functional GLP-1 receptor. Antagonism of the glucagon receptor resulted in compensatory overproduction of GLP-1, likely from the pancreatic α-cells, and this may play an important role in intraislet regulation and maintenance of β-cell function.
A promising area of diabetes research involves the use of small molecule antagonists, mixed agonists/antagonists, partial agonists, negative allosteric modulators or inverse agonists of the glucagon receptor to lower the level of circulating glucagon and thereby lower the glycemic level. Therapeutically, it is anticipated that inactivation of the glucagon receptor would be an effective strategy for lowering blood glucose by reducing hepatic glucose output and normalizing glucose stimulated insulin secretion. Consequently, a glucagon antagonist, mixed agonist/antagonist, partial agonist, negative allosteric modulator or inverse agonist may provide therapeutic treatment for NIDDM T2DM and associated complications, inter alia, hyperglycemia, dyslipidemia, insulin resistance syndrome, hyperinsulinemia, hypertension, and obesity.
Several drugs in five major categories, each acting by different mechanisms, are available for treating hyperglycemia and subsequently, NIDDM T2DM (Moller, D. E., “New drug targets for Type 2 diabetes and the metabolic syndrome” Nature 414; 821-827, (2001)): (A) Insulin secretagogues, including sulphonyl-ureas (e.g., glipizide, glimepiride, glyburide) and meglitinides (e.g., nateglidine and repaglinide) enhance secretion of insulin by acting on the pancreatic beta-cells. While this therapy can decrease blood glucose level, it has limited efficacy and tolerability, causes weight gain and often induces hypoglycemia. (B) Biguanides (e.g., metformin) are thought to act primarily by decreasing hepatic glucose production. Biguanides often cause gastrointestinal disturbances and lactic acidosis, further limiting their use. (C) Inhibitors of alpha-glucosidase (e.g., acarbose) decrease intestinal glucose absorption. These agents often cause gastrointestinal disturbances. (D) Thiazolidinediones (e.g., pioglitazone, rosiglitazone) act on a specific receptor (peroxisome proliferator-activated receptor-gamma) in the liver, muscle and fat tissues. They regulate lipid metabolism subsequently enhancing the response of these tissues to the actions of insulin. Frequent use of these drugs may lead to weight gain and may induce edema and anemia. (E) Insulin is used in more severe cases, either alone or in combination with the above agents.
Ideally, an effective new treatment for NIDDM T2DM would meet the following criteria: (a) it would not have significant side effects including induction of hypoglycemia; (b) it would not cause weight gain; (c) it would at least partially replace insulin by acting via mechanism(s) that are independent from the actions of insulin; (d) it would desirably be metabolically stable to allow less frequent usage; and (e) it would be usable in combination with tolerable amounts of any of the categories of drugs listed herein.
A number of publications have appeared which disclose non-peptide compounds which act at the glucagon receptor. For example, WO 03/048109, WO 2004/002480, WO 2005/123668, WO 2005/118542, WO 2006/086488, WO 2006/102067, WO 2007/106181, WO 2007/114855, WO 2007/120270, WO 2007/123581 and Kurukulasuriya et al. Bioorganic & Medicinal Chemistry Letters, 2004, 14(9), 2047-2050 each disclose non-peptide compounds that act as glucagon receptor antagonists. Although investigations are on-going, there still exists a need for a more effective and safe therapeutic treatment for diabetes, particularly NIDDM.